1. Field of the Invention
The present invention relates to an ion source used for, for example, doping of impurities, synthesis of materials, reformation of a surface, and development of a new material.
2. Description of the Related Art
FIG. 7 is a cross-sectional diagram showing a conventional ion source described in "Ion Source Engineering" written by Junzo Ishikawa and published from Ionics Inc. on May 31, 1986. In FIG. 7, 1 denotes a plasma chamber in which plasma 2 is generated. An opening 3 (hereafter, ion extraction port) is formed as part of the plasma chamber 1 to extract ion from the plasma 2 in the plasma chamber. An extraction electrode 4 is installed outside the plasma chamber and in the vicinity of the ion extraction port 3. An earthed electrode 5 is installed on the opposite side of the plasma chamber 1 relative to the extraction electrode 4. An accelerating power source 6 has a positive terminal connected to the plasma chamber 1 and a negative terminal which is grounded. An extraction power supply 7 has a negative terminal which is connected to the extraction electrode 4 and a positive terminal which is grounded. Then, 8 denotes an ion beam extracted from the plasma 2 in the plasma chamber 1.
Next, the operations of the prior art will be described.
An electric field is produced between the extraction electrode 4, to which a potential which is negative with respect to the one in the plasma chamber 1 is applied, and the plasma chamber 1. Due to the electric field, the ion beam 8 is extracted from the plasma 2 generated in the plasma chamber 1. At this time, positive spatial charges of ions themselves restrict an ion current I.sub.si (A) of the ion beam 8 extractable from the plasma 2 to a value represented as the expression (1), (refer to page 2 in the aforesaid "Ion Source Engineering") EQU I.sub.si =4.3.times.10.sup.-8 .times.(2a/d).sup.2 .times.M.sup.-0.5 .times.V.sup.1.5 r (1)
In the above expression (1), a represents a radius (cm) of the circular ion extraction port 3, and d represents a space (cm) between the ion extraction port 3 and the extraction electrode 4, V represents a potential difference (extraction voltage) (V) between the ion extraction electrode 4 and the plasma chamber 1, and r represents a coefficient (space charge limit relaxation coefficient) for relaxing limitation of an ion current I.sub.si extractable when positive space charges of ions are neutralized with negative space charges of electrons, and assumes any value larger than 1.
An ion current I.sub.pi the plasma 2 can supply is represented as the expression (2). EQU I.sub.pi =3.0.times.10.sup.-13 .times.a.sup.2 .times.(Te/M).sup.0.5 .times.ni (2)
In the above expression (2), T.sub.e denotes an electron temperature (eV) of the plasma 2, and ni represents a plasma density (cm.sup.-3).
When I.sub.si =I.sub.pi, the ion beam 8 is extracted optimally from the plasma 2 (refer to page 3 in the aforesaid "Ion Source Engineering").
According to the expression (1), the extraction voltage V is determined by use conditions of an apparatus (20 to 50 kV, in general, when an apparatus is employed for ion implantation). The electrode space d can be reduced merely to a value that does not cause discharge due to the extraction voltage V between the extraction electrode 4 and the plasma chamber 1 (for example, when the extraction voltage V is 40 kV, the extraction electrode space d is about 1 cm.). The ion extraction port 3 must be sized similarly to the electrode space d for optimal ion extraction. Therefore, when space charges of ions are not neutralized with electrons (space charge relaxation coefficient r=1), an extractable ion current I.sub.si is restricted by the above requirements. Assuming that the extraction voltage V is 40 kV, the extraction electrode space d is 1 cm, the diameter 2a of the ion extraction port 3 is 1 cm, and ionized gas is ionized argon gas (M=40), the above expression (1) is to be calculated. When the space charge limit relaxation coefficient r is 1, a maximum value for the extractable ion current I.sub.si is restricted to 54 mA. Therefore, even when plasma 2 from which a sufficiently large ion current I.sub.pi can be extracted is generated, if space charges of ions are not neutralized with electrons, an ion current I.sub.si of 54 mA or larger cannot be extracted.
To avoid the foregoing limitation of an extractable ion current I.sub.si, space charges of ions in the vicinity of the ion extraction port 3 must be neutralized with electrons so that the space charge limit relaxation coefficient r in the expression (1) will be larger than 1. A method has been revealed to radiate an electron beam 9 from the extraction electrode 4 toward the ion extraction port 3 as shown in FIG. 8 (refer to page 196 in the aforesaid "Ion Source Engineering"). According to this method, an effect of neutralizing space charges of ions is inversely proportional to the velocity of the radiated electron beam 9. Therefore, a lower velocity results in a greater neutralization effect. In other words, the lower the energy of the electron beam 9 is, the greater the neutralization effect becomes. In reality, however, the electron beam 9 from the ion attraction electrode 4 is accelerated due to an extraction electric field produced with an extraction voltage V induced between the extraction electrode 4 and the plasma chamber 1. Then, the electron beam 9 with high energy is radiated toward the ion extraction port 3. Therefore, space charges of ions are neutralized with space charges of ions ineffectively.
In a conventional ion source, as described above, an electron beam with high energy is radiated from an extraction electrode 4 toward an ion extraction port 3. Therefore, an electron beam 9 to be radiated to the ion extraction port 3 must have a current large enough to neutralize space charges of ions in the vicinity of the ion extraction port 3 and thus extract a large amount of ion current I.sub.si. However, the radiated electron beam 9 with a large current supplies current to an accelerating power source 6. This increases a load to be imposed on the accelerating power source 6. Furthermore, if the extraction voltage V is, for example, 40 kV, the electron beam 9 with a considerably large current of 100 mA is radiated toward the ion extraction port 3. Then, high power of at least 4 kW flows through the ion source. As a result, the ion source is heated. Unless a cooling mechanism is implemented, the ion source will be fused.